This figure shows a CT scan where the lung on the left is severely impacted by emphysema.
The region indicated by the arrow is clear because

Question

This figure shows a CT scan where the lung on the left is severely impacted by emphysema. 
The region indicated by the arrow is clear because

Quizplus · Accepted Answer

The clear region indicated by the arrow is due to the loss of alveoli in this area, which is a characteristic feature of emphysema. Emphysema is a chronic obstructive pulmonary disease (COPD) that is caused by exposure to irritants, such as cigarette smoke. As the disease progresses, the walls of the alveoli weaken and rupture, causing the larger air spaces seen on the CT scan. This results in reduced surface area for gas exchange in the lungs and impaired breathing function. A tumor or fluid accumulation would appear as a solid or cloudy region on the CT scan and tar would not cause such a clear region.

This Figure Shows a CT Scan Where the Lung on the Left